Murine and feline leukemia viruses have been produced in large quantities from continuous suspension cultures and the interspecies antigens isolated in highly purified form after passage through gel filtration columns. High titer monospecific antisera have been raised in rabbits against the interspecies components (p31) of both the feline (Rickard) and murine (Friend) virus lines. In addition, antisera against the three major polypeptides containing the species-specific components and having molecular weights of 10,000 (p10), 12,000 (p12) and 15,000 (p15) of murine and feline leukemia viruses have been prepared. The interspecies and species-specific antisera has been used for assay of immunologic competition in (a) primary and malignant urothelial and (b) epithelial cells obtained from tissue culture. Five of nine urothelial tumors have shown competition when tested with the anti-FeLV p31 sera. They have also been assayed against species-specific antisera of both murine and feline viruses and have shown no evidence of competition with any of these species-specific I125 labeled polypeptides. One of these tissues, successfully established as tissue culture, and which subsequently underwent spontaneous transformation in tissue culture, exhibited immunologic reactivity both before and after transformation. Six samples of non-malignant human urothelium obtained at surgery have been examined and none demonstrate any immunologic reactivity.